1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically to a current mirror type constant current source circuit which is mainly composed of MOS field effect transistors and which can be incorporated in a semiconductor integrated circuit.
2. Description of Related Art
A typical conventional current mirror type constant current source circuit includes a current mirror circuit, which is composed of a first n-channel MOS transistor having a gate and a drain short-circuited to each other, and a second n-channel MOS transistor having a gate connected to the gate of the first n-channel MOS transistor. The drain of the first n-channel MOS transistor is connected through a constant current source to a high level line of a voltage supply, and a source of the first n-channel MOS transistor is connected to a grounded line of the voltage supply. A source of the drain of the second n-channel MOS transistor is also grounded, and a drain of the second n-channel MOS transistor is connected to a load circuit so as to supply a constant current to the load circuit.
With the above mentioned arrangement, a current supplied from the constant current source flows through the first n-channel MOS transistor, and, a corresponding gate-source voltage appears between the gate and the source of the first n-channel MOS transistor. This gate-source voltage of the first n-channel MOS transistor is determined in accordance with the characteristics of the first n-channel MOS transistor, by the current supplied from the constant current source. The gate-source voltage of the first n-channel MOS transistor is applied between the gate and the source of the second n-channel MOS transistor, so that the second n-channel MOS transistor will allow to flow therethrough an output current, which is determined by the applied gate-source voltage in accordance with the characteristics of the second n-channel MOS transistor.
The above mentioned conventional current mirror type constant current source circuit has been disadvantageous in that when a voltage of the voltage supply increases, a current of the second n-channel MOS transistor supplied to the load circuit correspondingly increases, resulting in an increased consumption power.
A source-drain current of a MOS transistor has a positive dependence upon not only a gate voltage but also a source-drain voltage in a saturated region of the characteristics of the MOS transistor. In other words, even if the gate voltage is maintained at a constant level, if the source-drain voltage increases, the source-drain current correspondingly increases. In the above mentioned conventional current mirror type constant current source circuit, the first n-channel MOS transistor and the constant current source form a voltage division circuit between the high level line and the ground line of the voltage supply. Therefore, if the voltage of the voltage supply increases, the source-drain voltage of the first n-channel MOS transistor in the current mirror circuit correspondingly increases, and therefore, the source-drain current of the second n-channel MOS transistor in the current mirror circuit similarly increases.
Particularly, if the constant current source is formed of a p-channel MOS transistor, when the voltage of the voltage supply increases, a change amount of the source-drain voltage of the first n-channel MOS transistor and a change amount of the source-drain voltage of the p-channel MOS transistor are substantially equal to a change amount of the voltage supply. Therefore, with increase of the voltage of the voltage supply, a current of the p-channel MOS transistor and hence the current of the first n-channel MOS transistor are correspondingly increased. As a result, the output current of the second n-channel MOS transistor is increased by the amount in proportion to the amount increased of the current of the first n-channel MOS transistor, and also by the amount dependent upon an increase of the source-drain voltage of the second n-channel MOS transistor itself.